Relay protection method and apparatus against LC parallel circuit detuning faults

ABSTRACT

A relay protection method against LC parallel circuit detuning faults comprises the steps of: a relay protection device samples a current of a parallel LC, that is, a reactor and a capacitor, and samples a total current flowing through the whole LC; convert the current of the reactor into a current of an equivalent capacitor; calculate amplitudes of the current of the equivalent capacitor and a current of a realistic capacitor and calculate an amplitude of the total current flowing through the LC; calculate a current amplitude ratio of the equivalent capacitor to the realistic capacitor; and when the amplitude of the total current flowing through the LC is large enough, send an alarm signal or a trip after a setting time delay if the current ratio exceeds a preset upper and lower limit range. Also provided is a corresponding relay protection device.

FIELD OF THE INVENTION

The present invention belongs to the field of power systems, andparticularly, to a relay protection method and apparatus against LCparallel circuit detuning faults.

BACKGROUND OF THE INVENTION

A high voltage direct current (DC) power transmission system generatesmassive harmonic waves in a DC system and an alternating current (AC)system connected therewith, and in order to inhibit output of theharmonic waves at the DC side of a converter, a high voltage passivetype DC filter is mounted at the DC side. A common passive filtercomprises a mono-tuning filter, a dual-tuning filter, a tri-tuning and ahigh pass filter. In dual-tuning and tri-tuning circuits, there is aparallel LC (reactor and capacitor) circuit. The protection of theexisting dual-tuning and tri-tuning filters is mainly unbalanceprotection of the capacitor, differential protection of the filters,unbalance protection of two groups of filters having same parameters,impedance protection and voltage ratio protection, and there is nodetuning protection method special for the LC parallel circuit therein.Related content may refer to Detection Method Research on Detuning FaultElements of High Voltage/Ultrahigh High Voltage Tri-Tuning DC Filters(Huihui LUO, South China University of Technology, Master's Thesis,2011) and DC Filter Protection in Ultrahigh Voltage DC Systems (JifengWEN, Power System Automation, Volume 28, Journal 21, Pages 69-72, 2004).

In addition, in respect to a long-distance power transmission mode wherea serial capacitance compensator and the like are arranged in a powertransmission line, when parameters are inappropriate, a subsynchronousoscillation phenomenon may occur, and the steam turbine-generator shaftmay be damaged in serious cases, leading to torsion of the shaft. Inorder to inhibit the subsynchronous oscillation, one of main methods isto mount blocking filters at the high voltage side of the unittransformer, and the LC parallel circuits are mounted therein. An LCdetuning protection method of the blocking filter is mainly theunbalance protection of the capacitor and the out-of-limit protection ofan LC current ratio, with respect to a LC current out-of-limitprotection method, patent 201310032991.7 discloses a technical solution,but this method, in order to avoid system disturbance or protectionmaloperation during oscillation, has to expand the upper and lower limitrange of a current ratio as well as prolong a protection delay constantvalue, and these measures reduce the protection sensitivity and cannotprovide fast protection.

SUMMARY OF THE INVENTION

An objective of the present invention is to provide an improved methodand apparatus against LC parallel circuit detuning faults, which retainthe characteristics of simple wiring and reliability of the originalmethod, and also improve the stability of calculation results of acurrent ratio and raise sensitivity and rapidity of protection.

In order to achieve the objective, the present invention adopts thetechnical solution:

a relay protection method against LC parallel circuit detuning faultscomprises the following steps:

(1) sampling, by a relay protection device, a current of a parallel LC(a reactor and a capacitor), and sampling a total current flowingthrough the whole LC;

(2) converting the current of the reactor into a current of anequivalent capacitor;

(3) calculating amplitudes of the current of the equivalent capacitorand a current of a realistic capacitor and calculating an amplitude ofthe current flowing through the LC;

(4) calculating a current amplitude ratio of the equivalent capacitor tothe realistic capacitor; and

(5) when the amplitude of the total current flowing through the LC islarge enough, sending an alarm signal or a trip with outputting normalopen contact after a setting time-delay if the current ratio exceeds apreset upper and lower limit range.

Further, the step (2) specifically comprises: converting the current ofthe reactor into a current of an equivalent capacitor bydifferentiation, wherein the formula adopted is:

$\begin{matrix}{{i_{Ceq}(n)} = {\frac{n_{{CT},L}}{n_{{CT},C}}{\quad\left\lbrack {{{LC}\frac{{i_{L}(n)} - {2{i_{L}\left( {n - 1} \right)}} + {i_{L}\left( {n - 2} \right)}}{T_{s}^{2}}} + {{RC}\frac{{i_{L}(n)} - {i_{L}\left( {n - 1} \right)}}{T_{s}}}} \right\rbrack}}} & {{Formula}\mspace{14mu} 1}\end{matrix}$

wherein, i_(L) is the current of the reactor, i_(Ceq) is the current ofthe equivalent capacitor, n_(CT,L) and n_(CT,C) are a current conversionratio of current transformers (CT) of a reactor subcircuit and acapacitor subcircuit in the parallel LC circuit respectively, L is aninductance value of the reactor, R is an equivalent resistance value ofthe reactor subcircuit (R is very small usually and can be neglected), Cis a capacitance value of the capacitor and T_(s) is a sampling timeinterval, and n is a serial number of a discrete current signal.

Further, the step (3) specifically comprises: only calculating currentamplitudes of working frequency fundamental waves regarding the LCparallel circuit detuning protection of blocking filters, a samplingfrequency f_(s) being usually a value ranging from 1200 Hz-2400 Hz; andcalculating the current amplitudes of 12-order, 24-order and 36-orderharmonics waves regarding the LC parallel circuit detuning protectionfor dual-tuning and tri-tuning DC filters, a higher sampling frequencyf_(s) being required and being usually a value ranging from 4800 Hz-10kHz, T_(s) being equal to 1/f_(s).

Further, the method calculating the current amplitudes of the workingfrequency fundamental waves is a full cycle wave, short data windowFourier value algorithm, or a sine wave peak value detection algorithmadopting narrow band pass filtering.

Further, the step (5) specifically comprises: when the amplitude of thetotal current flowing through the LC is large enough, sending an alarmsignal or a trip signal by delay and outputting an idle contact of atrip delay if the current ratio exceeds a preset upper and lower limitrange, wherein an upper limit constant value Ratio_(set.max) of thecurrent ratio is Ratio_(nrml)+(2%˜10%)Ratio_(nrml), a lower limitconstant value Ratio_(set.min) of the current ratio isRatio_(nrml)−(2%˜10%)Ratio_(nrml), in which Ratio_(nrml) is a currentratio in a normal case and Ratio_(nrml)=1.0 theoretically.

Further, the setting time-delay value range is 0-60 s; and a fastprotection delay default range is 0.1-5 s.

The invention further provides a relay protection device against LCparallel circuit detuning faults, characterized by comprising a samplingmodule, a converting module, a calculating module and a judging andactivating module, wherein:

the sampling module is configured to sample, by a relay protectiondevice, a current of a parallel LC (a reactor and a capacitor), andsample a total current flowing through the whole LC;

the converting module is configured to convert the current of thereactor obtained by the sampling module into a current of an equivalentcapacitor; the calculating module is configured to calculate amplitudesof the current of the equivalent capacitor and a current of a realisticcapacitor according to results of the sampling module and the convertingmodule and calculate an amplitude of the current flowing through the LC,as well as calculate a current amplitude ratio of the equivalentcapacitor to the realistic capacitor; and

the judging and activating module is configured to, when the amplitudeof the total current flowing through the LC is large enough, send,according to a result of the calculating module, an alarm signal or atrip signal by delay and output an idle contact of a trip delay if thecurrent ratio exceeds a preset upper and lower limit range.

Further, the converting module converts the current of the reactor intoa current of an equivalent capacitor by differentiation, wherein theadopted formula for converting is:

$\begin{matrix}{{i_{Ceq}(n)} = {\frac{n_{{CT},L}}{n_{{CT},C}}{\quad\left\lbrack {{{LC}\frac{{i_{L}(n)} - {2{i_{L}\left( {n - 1} \right)}} + {i_{L}\left( {n - 2} \right)}}{T_{s}^{2}}} + {{RC}\frac{{i_{L}(n)} - {i_{L}\left( {n - 1} \right)}}{T_{s}}}} \right\rbrack}}} & {{Formula}\mspace{14mu} 1}\end{matrix}$

wherein, i_(L) is the current of the reactor, i_(Ceq) is the current ofthe equivalent capacitor, n_(CT,L) and n_(CT,C) are a current conversionratio of current transformers of a reactor subcircuit and a capacitorsubcircuit in the parallel LC circuit respectively, L is an inductancevalue of the reactor, R is an equivalent resistance value of the reactorsubcircuit (R is very small usually and can be neglected), C is acapacitance value of the capacitor and T_(s) is a sampling timeinterval, and n is a serial number of a discrete current signal.

Further, the calculating module calculating an amplitude of the currentflowing through the LC specifically comprises: only calculating currentamplitudes of working frequency fundamental waves regarding the LCparallel circuit detuning protection of blocking filters, a samplingfrequency f_(s) being usually a value ranging from 1200 Hz-2400 Hz; andcalculating the current amplitudes of 12-order, 24-order and 36-orderharmonics waves regarding the LC parallel circuit detuning protectionfor dual-tuning and tri-tuning DC filters, a higher sampling frequencyf_(s) being required and being usually a value ranging from 4800 Hz-10kHz, T_(s) being equal to 1/f_(s).

Further, the method calculating the current amplitudes of the workingfrequency fundamental waves is a full cycle wave, short data windowFourier value algorithm, or a sine wave peak value detection algorithmadopting narrow band pass filtering.

Further, the judging and activating module is configured to, when theamplitude of the total current flowing through the LC is large enough,send an alarm signal or a trip signal by delay and output an idlecontact of a trip delay if the current ratio exceeds a preset upper andlower limit range, wherein an upper limit constant value Ratio_(set.max)of the current ratio is Ratio_(nrml)+(2%˜10%)Ratio_(nrml), a lower limitconstant value Ratio_(set.min) of the current ratio isRatio_(nrml)−(2%˜10%)Ratio_(nrml), in which Ratio_(nrml) is a currentratio in a normal case and Ratio_(nrml)=1.0 theoretically.

Further, the setting time-delay value range is 0.5-5 s.

After the solution is adopted, the invention adopts a differentialcalculating method to convert the current of the reactor into thecurrent of the equivalent capacitor, then calculates a ratio of theamplitudes of the current, thereby retaining the characteristics ofsimple wiring and reliability of the original detuning protection,current out-of-limit protection method, and also improving the stabilityof calculation results of a current ratio and raising sensitivity andrapidity of protection.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a dual-tuning DC filter of a high voltage DC powertransmission system;

FIG. 2 shows a tri-tuning DC filter of a high voltage DC powertransmission system; in FIG. 2, a reactor L2 and a capacitor C2constitute a parallel circuit, and in FIG. 2, a reactor L3 and acapacitor C3 constitute a parallel circuit;

FIG. 3 shows a circuit schematic diagram of a generator, an unittransformer and blocking filters;

FIG. 4 is a wiring schematic diagram of an internal circuit of phase-Ablocking filters and current transformers;

FIG. 5 is a protection logic diagram of an embodiment of the presentinvention; and

FIG. 6 is a structural diagram of an apparatus of an embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The technical solution of the present invention is explained in detailin combination with drawings and specific embodiments.

An embodiment of the present invention provides a relay protectionmethod against LC parallel circuit detuning faults, comprising thefollowing steps:

(1) sampling, by a relay protection device, a current of a parallelreactor to obtain a current i_(L), sampling a current of a parallelcapacitor to obtain a current i_(C), and sampling a total currentflowing through the LC parallel circuit to obtain a current i_(LC);

(2) converting the current i_(L) into a current i_(Ceq) of an equivalentcapacitor by differentiation (or differential calculus);

(3) calculating amplitudes I_(Ceq), I_(c) and I_(LC) of the currentsi_(Ceq), i_(C) and i_(LC) by a full cycle wave short data window Fouriervalue algorithm, or a sine wave peak value detection algorithm adoptingnarrow band pass filtering;

(4) calculating a ratio of I_(Ceq) to I_(c); and

(5) when the amplitude I_(LC) of the current flowing through the LC islarge enough, sending an alarm signal or a trip signal by delay andoutputting an idle contact of a trip delay if the current ratio exceedsa preset upper and lower limit range.

The present invention provides the improved method based on the priorart, but the application occasions of the present method are not limitedto the blocking filter, and can be applied to the relay protection of LCparallel circuits such as a dual-tuning DC filter and a tri-tuning DCfilter of a DC power transmission system.

The following embodiment is taken for explaining: a 600 MW powergeneration set has a rated power 600 MW, a rated voltage 22 kV and arated power factor 0.9; and a main transformer has a rated capacity 750MVA, a rated voltage ratio 500 kV/22 kV, a Yd-11 wiring manner and ashort circuit impedance 13.5%. As shown in FIG. 3, a is a generator, bis the unit transformer, c is an phase-A blocking filter, and circuitsof phase-B and phase-C blocking filters are the same the circuit of thephase-A blocking filter.

The neutral point at the high voltage side of the unit transformer isopened and is serially connected to three phase static blocking filter(SBF) and is then grounded, wherein the SBF consists of an inductor anda capacitor in serial and parallel connection. A circuit topologicaldiagram is as shown in FIG. 4, in which:

PSW1 is a subcircuit switch, L0 is a 0 order reactor, C1 and L1 are afirst group of reactor and capacitor connected in parallel, C2 and L2are a second group of reactor and capacitor connected in parallel, C3and L3 are a third group of reactor and capacitor connected in parallel,CT_(C1) is a current transformer of C1, CT_(L1) is a current transformerof L1, and CT_(LC) is a current transformer of the whole blockingfilter. The current transformers of C2, L2, C3 and L3 are omitted in thedrawing and are not shown.

Basic parameters of the SBF are as follows:

The impedance of L0 is Z_(L0)=28.25Ω the impedance of the first group ofparallel reactor L1 is Z_(L1)=9.457Ω, the capacitive reactance of thefirst group of parallel capacitor C1 is X_(C1)=5.272Ω, the impedance ofthe second group of parallel reactor L2 is Z_(L2)=39.578Ω, thecapacitive reactance of the second group of parallel capacitor C2 isX_(C2)=91326Ω, the impedance of the third group of parallel reactor L3is Z_(L3)=20.054Ω, the capacitive reactance of the third group ofparallel capacitor C3 is X_(C3)=3.368Ω, a current conversion ratio ofthe current transformer CT_(L1) is n_(CT,L1)=1200 A/5 A, a currentconversion ratio of the current transformer CT_(C1) is n_(CT,C1)=2500A/5 A, and a current conversion ratio of the current transformer isn_(CT,SBF)=1000 A/5 A. The first group of parallel reactor and capacitorin FIG. 2 is taken as an example for explaining a specific implementingmethod of detuning fault protection. The implementing methods ofdetuning protection of the second and third groups of parallel reactorsand capacitors are the same as that of the first group of parallelreactor and capacitor, and the implementing methods of detuningprotection of phase-B and phase-C are same as that of detuningprotection of the phase-A.

The detuning protection of the phase-A first group of parallel reactorand capacitor comprises the specific implementing steps:

(1) step 1, sampling, by a relay protection device, a current of thefirst group of parallel reactor to obtain a current i_(L), sampling acurrent of a parallel capacitor to obtain a current i_(C), and samplinga total current flowing through the LC parallel circuit to obtaini_(LC), wherein a default sampling frequency is f_(s)=1200 Hz, the relayprotection device is only connected to a current, and wiring is simple;current amplitudes of working frequency fundamental waves are onlycalculated regarding the LC detuning protection of blocking filters, anda sampling frequency f_(s) is usually a value ranging from 1200 Hz-2400Hz; and in order to calculate the current amplitudes of 12-order,24-order and 36-order harmonics waves regarding the detuning protectionfor dual-tuning and tri-tuning DC filters, and the sampling frequencyf_(s) needs to be higher and is usually a value ranging from 4800 Hz-10kHz.

(2) step 2, converting the current i_(L) into a current i_(Ceq) of anequivalent capacitor by differentiation (or differential calculus),wherein the formula adopted is:

$\begin{matrix}{{i_{Ceq}(n)} = {\frac{n_{{CT},L}}{n_{{CT},C}}{\quad\left\lbrack {{{LC}\frac{{i_{L}(n)} - {2{i_{L}\left( {n - 1} \right)}} + {i_{L}\left( {n - 2} \right)}}{T_{s}^{2}}} + {{RC}\frac{{i_{L}(n)} - {i_{L}\left( {n - 1} \right)}}{T_{s}}}} \right\rbrack}}} & {{Formula}\mspace{14mu} 1}\end{matrix}$

wherein, n_(CT,L) and n_(CT,C) are a current conversion ratio of currenttransformers (CT) of a reactor subcircuit and a capacitor subcircuit inthe parallel LC circuit respectively, L is an inductance value of thereactor, R is an equivalent resistance value of the reactor subcircuit(R is very small usually and can be neglected), C is a capacitance valueof the capacitor and T_(s) is a sampling time interval, and n is aserial number of a discrete current signal. T_(s)=1/f_(s). In thisexample,

$L = {\frac{Z_{L\; 1}}{2\;\pi \times 50\mspace{14mu}{Hz}} = {\frac{9.457\;\Omega}{2\;\pi \times 50\mspace{14mu}{Hz}} = {0.03010H}}}$$C = {\frac{1}{2\;\pi \times 50\mspace{14mu}{Hz} \times X_{C\; 1}} = {\frac{1}{2\;\pi \times 50\mspace{14mu}{Hz} \times 5.272} = {6.038 \times 10^{- 4}F}}}$R = 0 Ω T_(s) = 1/f_(s) = 1/(1200  Hz) n_(CT, L) = n_(CT, L 1) = 1200/5n_(CT, C) = n_(CT, C 1) = 2500/5

(3) step 3, calculating amplitudes I_(Ceq), I_(c) and I_(LC) of thecurrents i_(Ceq), i_(C) and i_(LC) by a full cycle wave short datawindow Fourier value algorithm, or a sine wave peak value detectionalgorithm adopting narrow band pass filtering, wherein in thisembodiment, the full cycle wave Fourier value algorithm is adopted andthe formula is as follows:

$\left\{ {{{\begin{matrix}{{I_{m}(n)} = \sqrt{{a_{m}^{2}(n)} + {b_{m}^{2}(n)}}} \\{{a_{m}(n)} = {\frac{2}{N}\left\lbrack {\sum\limits_{k = 0}^{N - 1}{{i_{m}(k)}\cos\;\left( {k \cdot \frac{2\;\pi}{N} \cdot P} \right)}} \right\rbrack}} \\{{b_{m}(n)} = {\frac{2}{N}\left\lbrack {\sum\limits_{k = 0}^{N - 1}{{i_{m}(k)}{\sin\left( {k \cdot \frac{2\;\pi}{N} \cdot P} \right)}}} \right\rbrack}}\end{matrix}\mspace{14mu} m} = {Ceq}},C,{LC}} \right.$

wherein, the subscript m=Ceq,C,LC denotes “equivalent capacitor”,“capacitor”, and “parallel LC”; I_(Ceq), I_(C) and I_(LC) respectivelydenote amplitudes of the current i_(Ceq) of the equivalent capacitor,current i_(C) of the capacitor and the current/LC flowing through the LCparallel circuit; N is a data window length in Fourier filteringcalculation, and a default value is N=24; and P is a harmonic order;regarding the LC detuning protection of the blocking filters, P=1 andfundamental waves are only taken for calculating; and regarding the LCdetuning protection for dual-tuning and tri-tuning DC filters, P isequal to 12, 24 or 36 according to the working requirements of thefilters, that is, the amplitudes of 12-order, 24-order and 36-ordersubharmonic currents are calculated. In this example, N=24.

(4) step 4, calculating a current ratio sequence of I_(Ceq) to I_(C) byadopting the following formula:

$\begin{matrix}{{{Ratio}(n)} = \frac{I_{Ceq}(n)}{\max\left\{ {{I_{C}(n)},ɛ} \right)}} & {{formula}\mspace{14mu} 3}\end{matrix}$

wherein, ε is a small threshold set by a program in the relay protectiondevice and a divider is prevented from being 0. A value range of ε is0.5-2% of a rated second amplitude of the capacitor current transformer,and in a default case, the amplitude 1% is a default value of ε. Forexample, a second effective value of the current transformer CT_(C1) is5 A, and 1% of the amplitude, i.e., 0.0707 A, corresponding to 5 A canbe used as the default value of ε.

(5) step 5, adopting a following criterion regarding the first group ofparallel LC detuning protection:I _(LC)(n)>I _(set) and [Ratio(n)>Ratio_(set,max) orRatio(n)<Ratio_(set,min)]  Formula 4

wherein, I_(set) is a current open constant value, Ratio_(set.max) is anupper limit of a current ratio protection constant value, andRatio_(set.min) is a lower limit of a current ratio protection constantvalue. A constant value setting method is as follows:

(a) the amplitude of the current flowing through LC is calculated in arated operation working condition to obtain I_(LC,rated) and a setprotection constant value of 10-40% of the amplitude is taken, whereinfor this example, when the motor operates in a rated condition, thecurrent flowing through the blocking filter is:

$I_{rated} = {{\left( \frac{600\mspace{14mu}{MW}}{{\sqrt{3} \cdot 500}\mspace{14mu}{{kV} \cdot 0.9}} \right) \cdot \left( \frac{5\mspace{14mu} A}{1000\mspace{14mu} A} \right)} = {3.849\mspace{14mu} A}}$

and for example, 20% is taken for setting, a calculation result is:I _(set)=20%×3.849 A=0.770 A

(b) a current ratio of I_(Ceq) to I_(C) is Ratio_(nrml)=1.0, a ratio p %is taken, and a value of p is 2-10, the Ratio_(set.min) is set byreducing Ratio_(nrml) by p %, and Ratio_(set.max) is set by increasingRatio_(nrml) by p %.

For example, when p=5, a calculation result isRatio_(set.min)=Ratio_(nrml)(1−p%)=1.0×(1−5%)=0.95Ratio_(set.max)=Ratio_(nrml)(1+p%)=1.0×(1+5%)=1.05

in the protection criterion, referring to FIG. 5, wherein,

h, i. A current ratio criterion logic element in formula 4 outputs 1when the condition is met otherwise outputs 0.

j. The current criterion logic element in formula 4 outputs 1 when thecondition is met otherwise outputs 0.

d. “Or” gate logic operation.

e. “And” gate logic operation.

f. Constant time lag delay logic, a timer begins counting when an inputchanges from 0 to 1, delays to reach a tj moment and logically outputs1, and outputs 0 when the input remains unchanged or changes from 1 to0.

g. outputting a protection alarm or trip result.

When the condition in formula 4 is met, timing begins, when timingexceeds the setting time delay value t_(set), the relay protectiondevice sends a corresponding alarm signal or trip; if the protectioncriterion is not met, the timer is returned back to 0. The setting timedelay value is set according to an influence of various external faultsor grid oscillation, and the setting time delay value t_(set) can be avalue between 0.1-5 s.

The above method realizes detuning protection of the phase-A first groupof blocking filters, and is the same as the implementing method of thedetuning protection of the phase-A second and third groups of parallelreactors and capacitors. The implementing methods of the phase-B andphase-C detuning protection are the same as that of the detuningprotection of the phase-A. The implementing method of the detuningprotection of the LC parallel circuit in the dual-tuning and tri-tuningDC filters is also the same, only a higher sampling frequency isrequired to calculate a higher-order harmonic current, which is notrepeated here.

In a case of a fault in or out of the LC, a conventional current ratiocalculation result has obvious fluctuation. For example the fault in theLC (turn-to-turn short circuit fault of the reactor or short circuitfault of the capacitor), then after the fault, the current of thereactor and the current of the capacitor both have sub synchronouscurrent components, which lead to the fluctuation of a current ratiocalculation result, and it possibly causes that the LC detuningprotection is started and then returned, and the protection action isdelayed. In order to eliminate such influence, a protection constantvalue can be changed, the upper limit constant value of the currentratio is improved, the lower limit constant value of the current ratiois reduced, but as a result, the protection sensitivity is reduced andslight internal faults cannot be reflected.

For another example, for the external LC fault disturbance, a groundingfault occurs on a grid power transmission line, re-switching on issuccessful after the fault is removed, in this process, the current ofthe reactor and the current of the capacitor both have great change, thesubsynchronous oscillation current components are laminated, the currentratio calculation result is fluctuated for long term (lasting for 5-10s), and especially, in the initial stage of the external fault, thecurrent ratio calculation result is fluctuated very dramatically. Inorder to avoid the influence of the external fault, the protection delayconstant value has to be prolonged, which delays the protection action.

After the method of the present invention is adopted, although thecurrent during the internal and external faults changes, particularly,the subsynchronous oscillation currents are laminated, but the waveformsof the current of the reactor and the current of the equivalentcapacitor are consistent, which makes the change degrees of theamplitude of the current of the reactor and the amplitude of the currentof the equivalent capacitor consistent, therefore the current ratioresult is very stable. For example, for the large external faultdisturbance such as grounding fault on the grid power transmission line,the current ratio can be stable within 0.3-0.5 s. Therefore, theinvention has the beneficial effects: retaining the characteristics ofsimple wiring and reliability of the original detuning protectionmethod, i.e., the current ratio out-of-limit protection method, and alsoimproving stability of calculation results of a current ratio andraising sensitivity and rapidity of protection.

In addition, an embodiment of the present invention further provides arelay protection device against LC parallel circuit detuning faults, asshown in FIG. 6, which comprises a sampling module, a converting module,a calculating module and a judging and activating module, wherein:

the sampling module is configured to sample, by a relay protectiondevice, a current of a parallel LC (a reactor and a capacitor), andsample a total current flowing through the whole LC;

the converting module is configured to convert the current of thereactor obtained by the sampling module into a current of an equivalentcapacitor;

the calculating module is configured to calculate amplitudes of thecurrent of the equivalent capacitor and a current of a realisticcapacitor and calculate an amplitude of the total current flowingthrough the LC, as well as calculate a current amplitude ratio of theequivalent capacitor to the realistic capacitor; and

the judging and activating module is configured to, when the amplitudeof the total current flowing through the LC is large enough, send,according to a result of the calculating module, an alarm signal or atrip after a setting time delay if the current ratio exceeds a presetupper and lower limit range.

The above embodiments are merely intended to explain a technical idea ofthe present invention, and do not limit a protection scope of thepresent invention, and any changes made based on the technical solutionaccording to the technical idea of the present invention fall within theprotection scope of the present invention.

What is claimed is:
 1. A relay protection method against LC parallelcircuit detuning faults, comprising: (1) sampling, by a relay protectiondevice, a current of a parallel reactor, sampling a current of aparallel capacitor, and sampling a total current flowing through the LCparallel circuit; (2) converting the current of the reactor into acurrent of an equivalent capacitor; (3) calculating amplitudes of thecurrent of the equivalent capacitor and a current of a realisticcapacitor and calculating an amplitude of the current flowing throughthe LC parallel circuit; (4) calculating a current amplitude ratio ofthe equivalent capacitor to the realistic capacitor; and (5) when theamplitude of the total current flowing through the LC parallel circuitis large enough, sending an alarm signal or a trip after a setting timedelay if the current amplitude ratio exceeds a preset upper and lowerlimit range.
 2. The relay protection method against LC parallel circuitdetuning faults according to claim 1, wherein the step (2) furthercomprises: converting the current of the reactor into the current of anequivalent capacitor by differentiation, wherein the formula adopted is:$\begin{matrix}{{i_{Ceq}(n)} = {\frac{n_{{CT},L}}{n_{{CT},C}}{\quad\left\lbrack {{{LC}\frac{{i_{L}(n)} - {2{i_{L}\left( {n - 1} \right)}} + {i_{L}\left( {n - 2} \right)}}{T_{s}^{2}}} + {{RC}\frac{{i_{L}(n)} - {i_{L}\left( {n - 1} \right)}}{T_{s}}}} \right\rbrack}}} & \;\end{matrix}$ wherein, i_(L) is the current of the reactor, i_(Ceq) isthe current of the equivalent capacitor, n_(CT,L) and n_(CT,C) are acurrent conversion ratio of current transformers of a reactor subcircuitand a capacitor subcircuit in the LC parallel circuit respectively, L isan inductance value of the reactor, R is an equivalent resistance valueof the reactor subcircuit, C is a capacitance value of the capacitor andT_(s) is a sampling time interval, and n is a serial number of adiscrete current signal.
 3. The relay protection method against LCparallel circuit detuning faults according to claim 2, wherein the step(3) further comprises: only calculating current amplitudes of workingfrequency fundamental waves regarding the LC parallel circuit detuningprotection of blocking filters, a sampling frequency f_(s) being a valueranging from 1200 Hz-2400 Hz; and calculating the current amplitudes of12-order, 24-order and 36-order harmonics waves regarding the LCparallel circuit detuning protection for dual-tuning and tri-tuning DCfilters, a higher sampling frequency f_(s) being a value ranging from4800 Hz-10 kHz, T_(s) being equal to 1/f_(s).
 4. The relay protectionmethod against LC parallel circuit detuning faults according to claim 3,wherein calculating the current amplitudes of the working frequencyfundamental waves is a full cycle wave, short data window Fourier valuealgorithm, or a sine wave peak value detection algorithm adopting narrowband pass filtering.
 5. The relay protection method against LC parallelcircuit detuning faults according to claim 1, wherein the step (5)further comprises: when the amplitude of the total current flowingthrough the LC parallel circuit is large enough, sending the alarmsignal or the trip signal by delay and outputting an idle contact of atrip delay if the current amplitude ratio exceeds the preset upper andlower limit range, wherein an upper limit constant value Ratio_(set.max)of the current amplitude ratio is Ratio_(nrml)+(2%˜10%)Ratio_(nrml), alower limit constant value Ratio_(set.min) of the current amplituderatio is Ratio_(nrml)−(2%˜10%)Ratio_(nrml), in which Ratio_(nrml) is anormal current ratio in a normal case and Ratio_(nrml)=1.0.
 6. The relayprotection method against LC parallel circuit detuning faults accordingto claim 1, wherein a setting time delay value range is 0-60 s; and afast protection delay default range is 0.1-5 s.
 7. A relay protectiondevice against LC parallel circuit detuning faults, comprising asampling module, a converting module, a calculating module and a judgingand activating module, wherein: the sampling module is configured tosample, by a relay protection device, a current of a parallel reactorand a parallel capacitor, and sample a total current flowing through thewhole LC parallel circuit; the converting module is configured toconvert the current of the reactor obtained by the sampling module intoa current of an equivalent capacitor; the calculating module isconfigured to calculate, according to results of the sampling module andthe converting module, amplitudes of the current of the equivalentcapacitor and a current of a realistic capacitor and calculate anamplitude of the total current flowing through the LC parallel circuit,as well as calculate a current amplitude ratio of the equivalentcapacitor to the realistic capacitor; and the judging and activatingmodule is configured to, when the amplitude of the total current flowingthrough the LC parallel circuit is large enough, send, according to aresult of the calculating module, an alarm signal or a trip after asetting time delay if the current amplitude ratio exceeds a preset upperand lower limit range.
 8. The relay protection device against LCparallel circuit detuning faults according to claim 7, wherein theconverting module converts the current of the reactor into a current ofthe equivalent capacitor by differentiation, wherein the formula forconverting is: $\begin{matrix}{{i_{Ceq}(n)} = {\frac{n_{{CT},L}}{n_{{CT},C}}{\quad\left\lbrack {{{LC}\frac{{i_{L}(n)} - {2{i_{L}\left( {n - 1} \right)}} + {i_{L}\left( {n - 2} \right)}}{T_{s}^{2}}} + {{RC}\frac{{i_{L}(n)} - {i_{L}\left( {n - 1} \right)}}{T_{s}}}} \right\rbrack}}} & \;\end{matrix}$ wherein, i_(L) is the current of the reactor, i_(Ceq) isthe current of the equivalent capacitor, n_(CT,L) and n_(CT,C) are acurrent conversion ratio of current transformers of a reactor subcircuitand a capacitor subcircuit in the LC parallel circuit respectively, L isan inductance value of the reactor, R is an equivalent resistance valueof the reactor subcircuit, C is a capacitance value of the capacitor andT_(s) is a sampling time interval, and n is a serial number of adiscrete current signal.
 9. The relay protection device against LCparallel circuit detuning faults according to claim 7, wherein thecalculating module is further configured to: only calculate currentamplitudes of working frequency fundamental waves regarding the LCparallel circuit detuning protection of blocking filters, a samplingfrequency f_(s) being a value ranging from 1200 Hz-2400 Hz; andcalculate the current amplitudes of 12-order, 24-order and 36-orderharmonics waves regarding the LC parallel circuit detuning protectionfor dual-tuning and tri-tuning DC filters, a higher sampling frequencyf_(s) being a value ranging from 4800 Hz-10 kHz, T_(s) being equal to1/f_(s).
 10. The relay protection device against LC parallel circuitdetuning faults according to claim 9, wherein the the calculation moduleis further configured to: calculate the current amplitudes of theworking frequency fundamental waves as a full cycle wave, short datawindow Fourier value algorithm, or a sine wave peak value detectionalgorithm adopting narrow band pass filtering.
 11. The relay protectiondevice against LC parallel circuit detuning faults according to claim 7,wherein the judging and activating module is configured to, when theamplitude of the total current flowing through the LC parallel circuitis large enough, send the alarm signal or the trip signal by delay andoutput an idle contact of a trip delay if the current amplitude ratioexceeds a preset upper and lower limit range, wherein an upper limitconstant value Ratio_(set.max) of the current ratio isRatio_(nrml)+(2%˜10%)Ratio_(nrml), a lower limit constant valueRatio_(set.min) of the current amplitude ratio isRatio_(nrml)−(2%˜10%)Ratio_(nrml), in which Ratio_(nrml) is a normalcurrent ratio in a normal case and Ratio_(nrml)=1.0.
 12. The relayprotection device against LC parallel circuit detuning faults accordingto claim 7, wherein a setting time delay value range is 0-60 s; and afast protection delay default range is 0.1-5 s.